Please read what I wrote. Jer. 28:5 has יִרְמְיָה (Yirme*yah*) and
Jer. 7:1 has יִרְמְיָהוּ (Yirme*yahu*). As for your former comment,
yes, that should have been the case, but that's not what happened.
That's why there is no transliteration of the Tetragrammaton in the
Septuagint. Ever.

1 Answer
1

This turns, I think, on a question of how personal names worked in the time period. There may not be a simple answer to your question. As you've noticed, the name is a short phrase, and the terminal element is theophoric. Semantically, the two versions mean the same thing, and may well have been interchangeable in conversation, or variable in regional dialog. He might well have answered to either with equal alacrity. It might even be that the full name was reserved for formal occasions, and in common conversation he was, more or less, 'Jerry'.

In general, Biblical Hebrew is full of alternative forms for things that are lengthened or shortened. Names are nouns, they follow the patterns of nouns, which include these things.